1. Field of the Invention
This invention relates generally to any system using a robust orthogonal frequency division multiplexing (OFDM) transmission scheme which is capable of reliably carrying a plurality of services or programs (referred to collectively herein as services) within each channel of allocated bandwidth even in a selective channel fading environment.
2. Background of Related Art
Orthogonal frequency division multiplexing (OFDM) is a conventional technique for transmitting data symbols using mutually independent and separated radio frequency (RF) subcarriers. OFDM has several desirable properties, e.g., it simplifies or even eliminates equalization problems considerably, has graceful performance degradation, and because of the absence of equalization, can be lower in complexity.
In a traditional multi-service OFDM system respective groups of available subcarriers in a given channel are assigned to each supported service according to the service""s bandwidth needs. Therefore, OFDM symbols typically comprise two or more services each.
FIG. 4 shows a block diagram of a conventional OFDM transmission system.
In particular, in the transmitting portion 420 of the OFDM transmission system shown in FIG. 4, a data source containing source bits from all the different services or programs (e.g., three different digital audio broadcast (DAB) services or programs) is input to a bits to symbol mapper 402. The bits to symbol mapper 402 maps the data bits for current data output from respective DAB services or programs into a contiguous symbol stream. The contiguous symbol stream is input to an OFDM transmitter and radio frequency (RF) transmitter front end 404, which transmits the contiguous symbol stream using the assigned portion of the available OFDM subcarrier frequencies.
At the receiving portion 430 of the OFDM transmission system shown in FIG. 4, an RF receiver front end and OFDM receiver 406 receives the contiguous symbol stream containing the information for the number of different DAB services or programs (e.g., for three different DAB services or programs). A symbols to bits demapper 408 converts the contiguous symbol stream back into a data bit stream containing data for all of the different DAB services or programs.
FIG. 5 shows a conventional frequency distribution of OFDM subcarriers 1 to N for use by the different DAB services or programs, e.g., three different DAB services or programs 502-506. The OFDM subcarriers 1 to N represent in this example all of the available subcarriers used in the transmission between the transmitting portion 420 and receiving portion 430 of a conventional OFDM transmission system, e.g., as shown in FIG. 4.
When a number of different DAB services or programs 502-506, e.g., three, are simultaneously transmitted, the available OFDM subcarriers 1 to N are conventionally distributed in frequency among the plurality of services 502-506. For instance, the available OFDM subcarriers 1 to N are typically assigned in fixed, contiguous groups with respect to frequency between the three different DAB services or programs 502-506 as depicted in FIG. 5. The number of subcarriers assigned to each DAB service or program is application specific, and typically depends upon the information capacity required by each DAB service or program.
Thus, in the disclosed example, a first DAB service or program 502 is assigned the first contiguous group N1 of all available OFDM subcarriers (e.g., 1 to N1), a second DAB service 504 is assigned the next contiguous group N2 of the remaining available OFDM subcarriers (e.g., N1+1 to N1+N2), and the third DAB service or program 506 is assigned the last contiguous group N3 of all available OFDM subcarriers (e.g., N1+N2+1 to N).
However, a transmission channel 540 (e.g., an FM station) containing the OFDM subcarriers 1 to N of the different DAB services or programs 502-506 may be subject to selective fading affecting some of the OFDM subcarriers but not others. This is particularly true in a fixed or slow speed mobile environment. In such a case, a large number of the assigned subcarriers of one DAB service or program may be detrimentally affected while at the same time all or most of the assigned subcarriers of other DAB services or programs may be unaffected.
This example is further illustrated in FIGS. 6 and 7 demonstrating the effect of selective fading on some but not all OFDM subcarriers.
In particular, in FIG. 6, at least sixty subcarrier frequencies 1 to 60 are assumed to be available in each data frame in the frequency domain. In the disclosed example, the first service 502 has the first ten (10) OFDM subcarrier frequencies 1 to 10 assigned thereto, the second service 504 has the next twenty (20) subcarriers 11 to 30 assigned thereto, and the third service 506 has the last thirty (30) subcarriers 31 to 60 assigned thereto.
Each of the different DAB services or programs 502-506 may conventionally have its own interleaver and/or Forward Error Correction (FEC) scheme to improve the quality of the transmission channel 540.
FIGS. 7A and 7B show a relevant portion of a possible fading scenario in the example shown in FIG. 6 in which five of the subcarriers of the first service 502 are hit by a frequency selective fade. In such a case, the fourth through ninth subcarriers 4 to 9 assigned to the first service 502 are shown as detrimentally affected by the selective frequency fade and likely lost. This loss of xc2xd(i.e., five (5) out of the ten (10) subcarriers or 50%) of the total subcarriers assigned to the first service 502 may be beyond the error recovery capability of the FEC scheme used for that particular service. In this example, however, the other two services 504 and 506 do not suffer from the frequency selective fades. Thus, while the symbols transmitted by the first service 502 during that use or data frame of the available subcarriers would likely be lost, the symbols transmitted by the other services 504 and 506 would be unaffected.
While the interleaver and/or FEC function of the various services or programs may be suitable to maintain reliable communications in the transmission channel 540, e.g., an FM station in the absence of selective fading, the interleaver and FEC function of the one service or program may not be adequate to fully overcome the deterioration of the data communication due to channel fades, particularly when they affect a large percentage of the assigned subcarrier frequencies for just one service or a small percentage of a large number of requesting services for the channel. Thus, the impact of channel fades may affect some services more than other services.
Accordingly, in a frequency selective environment, one or more of the services may be rendered useless at the receiver due to channel impairments. There is thus a need to improve the reliability of all or substantially all services or programs transmitted using OFDM modulation techniques within a transmission channel, e.g., within an FM channel.
In accordance with the principles of the present invention, a time division multiplexed orthogonal frequency division multiplexed transmitter comprises a bits to symbols mapper, a time division multiplex symbol rearranger in communication with the bits to symbols mapper, and an OFDM transmitter and radio frequency front end in communication with the time division multiplex symbol rearranger.
In another aspect of the present invention, a time division multiplexed orthogonal frequency division multiplexed receiver comprises a radio frequency front end and OFDM receiver, a time division multiplex symbol arranger in communication with the radio frequency front end and OFDM receiver, and a symbols to bits demapper in communication with the time division multiplex symbol arranger.
A method of transmitting symbols relating to a plurality of services in accordance with the principles of the present invention comprises assigning respective periods of time for transmission to each of a plurality of services. At least one symbol is firstly transmitted for a first one of the plurality of services using substantially all available subcarrier frequencies in an orthogonal frequency division multiplexed transmission system for a first period of time assigned to the first one of the plurality of services.
A method of transmitting symbols relating to a plurality of services in accordance with another aspect of the present invention comprises assigning respective periods of time for transmission to each of a plurality of services, and transmitting data from less than all of the plurality of services in a data frame corresponding to a use of all of a plurality of available subcarrier frequencies.